Two-piece golf ball

Abstract: Disclosed is a two-piece golf ball comprising a core comprising a rubber composition of a base rubber, a co-crosslinking agent and an organic peroxide, and a cover. The core of the golf ball contains 20 to 30 parts by weight of the co-crosslinking agent based on 100 parts by weight of the base rubber. The hardness of the core, measured by a JIS-C type hardness tester, is within a range of from 70 to 80 at any part from the center to the surface and a difference in hardness at any part is not more than 5. The amount of compression deformation of the core formed between initial loading (10 kg) and final loading (130 kg) is 3.1 to 3.8 mm.

Description: The present invention relates to a two-piece golf ball. More particularly, it relates to a two-piece golf ball having excellent shot feel. A two-piece golf ball has widely been used because of its excellent flight performances. However, there is a problem that shot feel of the two-piece golf ball is hard in comparison with a thread wound golf ball and it exhibits a harder feeling when it is mis-hit. Therefore, two-piece golf balls having an excellent shot feel are a desiratum. Recently, various efforts have been made in order to obtain a shot feel which is similar to that of the thread wound golf ball. For example, the shot feel may be improved by softening the core of the two-piece golf ball to decrease the total hardness of the ball. However, the golf ball obtained by this method lacks a suitable hardness distribution in the core. Further, the golf ball feels soft but exhibits a heavy feeling and, therefore, it does not necessarily have a shot feel which is similar to that of the thread wound golf ball. GB-A-2127303 relates to a two-piece solid golf ball. Under these circumstances, the present inventors have paid attention to the hardness distribution of the core and compression strength and studied intensively about them. As a result, it has been found that, by making the hardness distribution of the core uniform within a range from the center to the surface, adjusting the compression strength of the core and optimizing both of these, it has become possible to obtain a suitable softness of the core and a suitable hardness of the cover, thereby affording a two-piece golf ball which is soft and superior in shot feel and which has excellent flight distance in comparison with a conventional golf ball. As a matter of course, the resulting two-piece golf ball has a long flight distance when hit with an iron and has excellent control properties. That is, the main object of the present invention is to provide a two-piece golf ball having a shot feel which closely approaches that of a thread wound golf ball by controlling the characteristics of the core of the two-piece golf ball. This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description. The present invention provides a two-piece golf ball comprising a core comprising a rubber composition of a base rubber, a co-crosslinking agent and an organic peroxide, and a cover covering the core, the core containing 20 to 30 parts by weight of the co-crosslinking agent based on 100 parts by weight of the base rubber, wherein the hardness of the core, measured by a JIS-C type hardness tester, is within a range of from 70 to 80 at any part from the center to the surface, the difference in hardness at any part being not more than 5, and wherein the compression deformation of the core, between an initial loading (10 kg) and a final loading (130 kg), is 3.1 to 3.8 mm. In the present invention, as the base rubber, there can be used natural rubbers and/or synthetic rubbers which have hitherto been used for a two-piece core. Particularly, a cis-1,4-polybutadiene rubber having at least 40% of a cis-structure is preferred. If necessary, natural rubber, polyisoprene rubber, styrene-butadiene rubber, EPDM, etc. may be suitably added to the above polybutadiene rubber. The co-crosslinking agent is not specifically limited, and examples thereof include metal salts of unsaturated carboxylic acids, particularly monovalent or divalent metal salts of unsaturated carboxylic acids having 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.). Among them, zinc acrylate is particularly preferred. The amount of the co-crosslinking agent is preferably 20 to 30 parts by weight based on 100 parts by weight of the base rubber. When the amount is smaller than 20 parts by weight, the hardness of the golf ball becomes low and an excessively heavy and inferior feel is obtained. Further the durability also becomes inferior. On the other hand, when the amount exceeds 30 parts by weight, the golf ball becomes too hard and an inferior feel is obtained. Examples of the organic peroxide include dicumyl peroxide, di-t-butyl peroxide and the like. Among them, dicumyl peroxide is particularly preferred. The amount of the organic peroxide is 0.5 to 5.0 parts by weight, preferably 0.8 to 3.0 parts by weight, based on 100 parts by weight of the base rubber. When the amount is smaller than 0.5 parts by weight, the hardness of the golf ball becomes low and an excessively heavy and inferior feel is obtained. On the other hand, when the amount exceeds 5.0 parts by weight, the golf ball becomes too hard and an inferior feel is obtained. If necessary, additives such as fillers, antioxidants, etc. may be added to the rubber composition used in the golf ball of the present invention. Examples of the filler include zinc oxide, barium sulfate and the like. An amount of the filler varies depends on a specific gravity, size, etc. of the cover and core and is not specifically limited, but is preferably 10 to 40 parts by weight based on 100 parts by weight of the base rubber. The core of the two-piece golf ball of the present invention is produced by heating at a temperature, at which a peak of temperature rise of the center part due to internal heat generation appears after 20 minutes has passed since the beginning of heating. After 20 minutes or more to adjust, the hardness (measured by a JIS-C hardness tester) at the center part is 70 or more. Heating is then continued at the same or higher temperature to adjust the hardness (measured by a JIS-C hardness tester) at the surface part from 70 to 80. By using this method, the hardness of the resulting golf ball falls within the range of from 70 to 80 with a difference in hardness at any part being not more than 5. The terms "difference in hardness at any part being not more than 5" means that a difference in hardness measured at any part of the core between maximum value and minimum value is within 5. Because of indefinite factors such as error of measurement, etc., the limitation "within 5" is not severe and it is understood that some deviation may exist. When the heating is conducted at a temperature at which a peak of temperature rise of the center part due to internal heat generation appears before 20 minutes has passed since the beginning of heating, internal heat regeneration is liable to arise rapidly causing heat deterioration. Further, when the heating time is smaller than 20 minutes, the internal vulcanization becomes insufficient. When the hardness is lower than the above range, the durability deteriorates and too soft and heavy feeling is obtained. When the hardness is higher than the above range, an impact force at the time of hitting is large and an inferior feel is obtained. In the present invention, it is necessary that an amount of compression deformation of the core formed between initial loading (10 kg) and final loading (130 kg) is 3.1 to 3.8 mm. The amount of compression deformation can be controlled mainly by changing the amount of the above metal salts of unsaturated carboxylic acids, but it can also be controlled by the amount of the other chemicals, vulcanization conditions, etc. Even if the amount of compression deformation is controlled by any one of the above methods, the resilience coefficient is decreased and flight performances are deteriorated when the amount of deformation of the core exceeds 3.8 mm. The resulting ball is too soft and exhibits heavy and unsatisfactory feel and the durability is also low. On the contrary, when the amount of deformation is smaller than 3.1 mm, hard and an inferior feel is obtained. A two-piece golf ball can be obtained by covering a cover on the core for golf ball thus obtained. The thickness of the cover is preferably 1.5 to 3.0 mm. As the cover, there can be normally used those which contain an ionomer resin as a base material and, if necessary, fillers (e.g. titanium dioxide, barium sulfate, etc.) to be added for the purpose of coloring. When the thickness of the cover becomes smaller than 1.5 mm, an excessive amount of spin is put on the golf ball and flight performances become inferior. Further, it becomes too soft and unsatisfactory in view of the feel obtained, thereby approaching the feel of the one-piece golf ball, and this is not preferred. On the other hand, when the thickness exceeds 2.5 mm, hard, heavy and inferior feel is obtained. A method of coating an ionomer resin on the cover is known to the person skilled in the art, and it is normally conducted by an injection molding. It is preferred that the contact area obtained when the golf ball according to the present invention is hit with a golf club at a head speed of 45 m/second is 4.3 to 5.0 cm 2 . When the contact area is smaller than 4.3 cm 2 , the contact surface with respect to the club is too small and control properties become inferior. Further, the hit feel is hard and an inferior feel is obtained. On the other hand, when the contact area exceeds 5.0 cm 2 , a resiliency becomes inferior and an inferior feel is obtained, which is not preferred. As described above, the hit feel of the two-piece golf ball of the present invention is similar to that of the thread wound golf ball and flight performance of the two-piece golf ball per se are maintained. Thus, the two-piece golf ball of the present invention is superior in flight performance hit feel and control properties. The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Examples 1 to 3 and Comparative Examples 1 to 2 The formulation components of the core shown in Table 1 were kneaded to prepare rubber compositions respectively. The rubber composition was subjected to a vulcanization molding in a mold under conditions shown in Table 1. The hardness distribution and the compression strength of the core thus obtained are shown in Table 1. The cover obtained from the formulation components of the cover according to a normal method was coated on the core. The stiffness and the thickness of the cover are shown in Table 1. The hardness (PGA indication), the durability index, resilience coefficient, the flight performances (e.g. launch angle, spin, carry, etc.), the contact area and the evaluation of feeling of the resulting two-piece golf ball are shown in Table 2. As is apparent from the results of Tables 1 and 2, the golf balls of Examples 1 to 3 of the present invention are light and soft and superior in hit feel and they are extremely similar to a thread wound golf ball. Further, they are superior in durability and flight performances. Regarding the golf ball of comparative Example 1, the compression strength of the core is large and the amount of deformation is small (2.85 mm). Further, the hardness of the golf bail is high, and its feel is too hard and heavy and inferior. Regarding the golf ball of Comparative Example 2, the compression strength of the core is small and the amount of deformation is large (4.10 mm). Further, the hardness of the golf ball is low, and its feeling is soft and heavy and inferior in resiliency. The durability is also inferior. The golf ball of Comparative Example 3 corresponds to the golf ball of Example 2 of Japanese Patent Laid-Open Publication No. 5-123422, but the resilience coefficient is low (energy loss is large), flight performances are inferior and the center part of the core is soft. Therefore, the feel is inferior and the resiliency becomes inferior.

Claims: A two-piece golf ball comprising a core comprising a rubber composition of a base rubber, a co-crosslinking agent and an organic peroxide, and a cover covering the core, the core containing 20 to 30 parts by weight of the co-crosslinking agent based on 100 parts by weight of the base rubber, characterised in that the hardness of the core, measured by a JIS-C type hardness tester, is within a range of from 70 to 80 at any part from the center to the surface, the difference in hardness at any part being not more than 5, and in that the compression deformation of the core, between an initial loading of 10 kg and a final loading of 130 kg is in the range of from 3.1 to 3.8 mm. A two-piece golf ball as claimed in claim 1, wherein a contact area obtained when the golf ball is hit with a club at a head speed of 45 m/second is in the range of from 4.3 to 5.0 cm 2 . A two-piece golf ball as claimed in claim 1 or claim 2 wherein the base rubber is a cis-1,4 polybutadiene rubber having at least 40% of a cis-structure. A two-piece golf ball as claimed in any one of claims 1 to 3 wherein the co-crosslinking agent is a monovalent or divalent metal salt of an unsaturated carboxylic acid having in the range of from 3 to 8 carbon atoms. A two-piece golf ball as claimed in claim 4 wherein the co-crosslinking agent is zinc acrylate. A two-piece golf ball as claimed in any one of claims 1 to 5 wherein the organic peroxide is dicumyl peroxide. A two-piece golf ball as claimed in any one of claims 1 to 6 wherein the organic peroxide comprises 0.5 to 5.0 parts by weight based on 100 parts by weight of the base rubber. A two-piece golf ball as claimed in any one of claims 1 to 7 further comprising a filler, antioxidant or a mixture thereof. A two-piece golf ball as claimed in claim 8 wherein the filler comprises zinc oxide, barium sulfate or a mixture thereof. A two-piece golf ball as claimed in claim 8 or claim 9 wherein the filler comprises 10 to 40 parts by weight based on 100 parts by weight of the base rubber.

Inventors: ENDO SEIICHIRO, HORIUCHI KUNIYASU, ENDO, SEIICHIRO, HORIUCHI, KUNIYASU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37